A problem concerned with previous tightening methods for pretensioning two or more screw joints at the same time is to obtain a balanced and even clamping force build-up between the parts being joined by the screw joints. One such method is described in Japanese Patent Application No.: JP 900335746 (Publication No.: JP 4201022). According to this know method, all joints are run down in a first step to a seated condition. Thereafter, the pretensioning phase is started for all joints, and during an initial stage, the individual torque growth characteristics for the joints are determined followed by an adjustment of the rotation speed of each spindle so as to aim at a simultaneous arrival of the screw joints to the desired torque level.
This known method for tightening process control is disadvantageous an that it is too slow, not only due to the tightening process being carried out in two steps, but also due to the torque growth adaptation by speed adjustment being too slow. The control of the process is also insufficient due to the speed adjustment being carried out only once. This may be sufficient to obtain some compensation at ideal joints having linear torque growth characteristics, but will result in unacceptable differences in tightening time for reaching a common predetermined torque or clamping force level at joints having different nonlinear torque rates, i.e. different torque/angle characteristics. This in turn would result in an undesired uneven claping force application on critical machine parts like shaft bearing caps and cylinder heads of internal combustion engines.
Another drawback concerned with this and other known methods for simuitaneous tightening of two screw joints is the risk for damage to the equipment and injury to the operator in case one of the two output spindles loosens its grip on the screw head or nut being tightened, because if the reaction support for one of the spindles is lost the entire nutrunner is exposed to the reaction torque from the other spindle. This is a safety risk for the operator who has to counteract the reaction torque by muscle force.